Radar tester



June 2l, 1960 Filed Dec. l2, 1955 R. c. HUNTINGToN 2,942,257

RADAR TESTER 2 Sheets-Sheet 1` EHDE OPERATOR.

FIG. I

EFL Ec Toe VOLTAGE I I I I S* Wesens/N6 A/Eqnr/VELY I I F IG. 5 I k MECH. Serf/NG, ".a." y Mean. 5ET7'M/e,"b e4 e@ eg I t IS w co 94a* '$3 *E o O 90d s gf b 96a.

a. S g Q Il) Lu L I L g g l l EEFLECTOE VOLTQGE D 91m d 96 Ve Ve V3 /az Ec Tore VOLT/96E -MECHQA//CQL Serf/uq b" INVENTOR. ,6085er c. #MvT/,VG To# ,4770 A/E Y June 21, 1960 R. c. HUNTINGTON RADAR TESTER #freek/EV' United States Patent TESTER Robert C. Y Mills, Inc., a corporation of Delaware Filed Dec. iz, 195s, ser. No. 552,518

i 30 Claims. (Cl. 343-173) This invention .relates to pulse echo systems andr pertains more particularly to apparatus for ascertaining Whether such systems are functioning properly.

One object of the instant invention is to provide a radar tester that is capable of ascertaining in a rapid manner the overall degree of operational tness of ythe radar Set undergoing test.

Another object of the invention is to provide sucient information so that the operator can diagnose the cause of system malfunctioning, should such be the case.

More specically, in the realization of the foregoing aims the tester is equipped to measure the radar transmitted power and the radar receiver sensitivity. In this regard it is contemplated that the radar testser be located in front of the radar set and just outside the Fresnel zone thereof. By means of a receiving horn antenna forming a part of the tester the radar signal is received andan indication of its amplitude obtained. After a suitable time delay, which delay when accurately controlled simulates the range of a target, the tester generates lan output signal of known power which is transmitted Vto the radar set through a separate transmitting horn. The

received signal measurement is in eiect a measurement` of the radar transmitted pulse peak power and the controlled measured output signal provides for a determination of the radar receiver sensitivity. f

A further object of the invention is to provide a time delay between the received and the transmitted signals that is adjustable to such an extent that a target range of from between ve and twenty miles can be simulated.

Also, it is an aim to vary the time delay at a continuous l Huntington, St. Paul, Minn., ass'gnor to General rate so that target speed is simulated, such speed falling within the rate range of from between 200 to 600 knots.

These ranges, of course, are only exemplary. U

Another object of the invention is to provide a tester of the foregoing type that will be very simple in its operation,rrequiring relatively inexperienced personnel. Furn ther, it may be explained that the system requires the employment of only one oscillator tube rather than two. For instance, when klystron or magnetron vtubes are employed as the oscillator tube, only onesuchtubeis needed.

Further, another object of the invention isto provide testing apparatus of this character that will bev exceptionally reliable in its operation over fairly long periods g of time without requiring extensive re-calibration of the various components comprising the system.

A still further advantageous featureV residesA in the fact that a testing system constructed in accordance with the teachings of the present invention requires no physical connection to the radar set, thereby obviating' any need for revamping the radar set to permit its testing and of course permitting ready use of the testing equipment for different types of radar installations.

Yet another object of the invention is to' provide apparatus for testing radar that will be portable in nature,

2,942,257 v Patented June 2l, 1960 ICC instance, it is a desideratum of lthe inventon'toutilize i the equipment for the testing of radarY installed'on various types of aircraft, it being possible tofmove the radar tester as a unit to a vantage point in'prximity to the particular craft that is to have its radar set tested.Y

Other objects will be in part obvious, and in part pointed out more in detail hereinafter.

The invention accordingly consists in the features of construction, combination of elements and arrangement of parts which will be exemplified in the construction hereafter set forth and the scope of the application which will be indicated in the appended claims.

In the drawings:

Figure 1 is a schematic lay-out of the radar tester system in actual use;

Fig. 2 shows the radar tester and Figs. 3, 4, and 5 illustrate various curves relating to the klystron tuning circuit. Y

Referring first to Fig. l, the lay-out view there presented is intended to give a general idea as to how the radar tester is to be employed. Accordingly, the radartester has been designated in its entirety bythe reference numeral 10, it receiving power from a source' or supply labelled 12. V'I'he tester 10 is located in front of* a parked aircraft I14 just outside of the Fresnel zone of a radar set 16 mounted in the nose of said aircraft. Intermediate the radar tester and the parked aircraft 14 is a microwave absorbent barrier 18 for the purpose of eliminating all ground rellections so that the radartes'ter 10 will receive only a direct signal from the radar set-16. In carrying out the test, two technicians are required, one being stationed in the aircraft 14 and one at the radar tester 10. These two technicians are in constantcommunication with each other via a teld telephone system20.

With the above described arrangement in mind, .attention is nowr directed-to the block -diagram set forth in Fig. 2. From this diagram it can'be seen that-theradar tester 10 includes an antenna 22 for receiving a radar pulse signal transmitted by the radar set 16 (Fig. V1). The antenna 22 feeds its signal directly to `aiixed attenuator 24 coupled to a balanced mixer 26. The balanced mixer is a standard type of veither the hybrid tee .or short-slotsvariety..A After'attenuation, the radar pulse signal is mixed with a continuouswave output froman automatic frequencyv controlled local oscillator circuit-to be described hereinafter. The local oscillator frequency is selected so that the output of the-mixer 26 Willbe'a forty megacycle intermediate frequency-pulse, or more precisely, a train of such pulses which are fed to a gainstableLF. preamplifier 28. t

The output from the preamplifier 28 is channelled in twoY directions as far `as the circuitry of the radar tester is concerned. We will deal with one course at. the present time, this course being by way of a calibrated vvariable attenua-tor designated by the numeral 30. More willbe in block diagram form,

'said laterfon concerning the duties of Ithis at-tenuator;

` to -a second Vgain-stable 11F. amplifier strip 32. Both of allowing its use under a variety of circumstances. `For the amplifiers 28 and 32 are conventional ksrtalggered-.tuned ampliltie'rswith additional gain-stability achieved bymeans of a special power supply which controls the-plate and screen voltage in order -to compensateffonchanges inthe heater voltages. 'The signal then Aproceeds to adetector `v 34 whose output will bea pulse-having a width equal to that of the radar signal pulse andof anamplitudeprolportional-to the peak of the received radar signal.

It is actually the amplitude ofV the vpulse produced by the detectordtlithat Ywe are primarily-concerned with and in this regard it is to be explained that the amplitude of this detector video pulse is to be compared with a preset known reference voltage. To do this, the video "pulse is fedto Lan `amplitudecomparison circuit 36v which 'is an amplitude discriminating circuit of the 'multiar type 'in which the input video pulses 'as received from the detector produce "a large output pulse byV regenerative fao-42,25?

action when `and only lwhen the "amplitude of the 'input `1f ulse`exceeds the-fixed reference voltage. Thediscrimination level is, of course, within the linear `operation of 'the LF. ampliers 28 and 32.

`As indicated above, comparison .is -to be made 'of the video output pulse from the detector 34 with a reference Y voltage and "the source or supply for-this 'reference volt age has been indicated "by 'the'mnm'eral 38. 'The value of the reference voltage, a D.C. voltagefis susceptible to variation `according to circuit designconsiderations. In the present situation, however, the voltage lwill be 'assurn'ed to Abe in 'the neighborhood of I1 to 1:5 volts. l"Assuming that we have selected 1.5 volts, the amplitude 'comparison ci-rcuit'functions in affashiin such that "it will generate its output pulse onlyf when the incoming-pulse Aarnplitudee'xceeds the 1.5 volt reference voltage. There- "fore, for detector pulses greater than Athe reference voltage, the amplitude `comparison *circuit pulse output wil-l -be available-for further utilization.

`Continuing with the description, Vit is 'to be lpointed 'outthat the output'pulsegeuera'ted by the amplitude comparison circuit is fed to a variable electronicdelay cir- -cuit `40 'composed of `a univibrator, a Miller integrator, `and Aasecond 'multiar circuit. Mor'e will be saidpresently `concerning 'the variable nature of the delay circuit 40. At'this time, "however, it is of 'interest to -rnerely mention that the vmountof delayintroduced by this circuit Vis accurately 4controlledsothat the resulting delay represents or simulates the 'range 'of a target. The delayed output -pulse from the circuit 40 is fed to a blocking oscillator `42 4capable 'of `gener-ating a pulse having 'a time duration corresponding to that of the originally received "radar pulse.

The pulse generated by the blocking oscillator 42 serves twopurposes, 'the 'rstof which will now be dealt with. In this "regard, it is to be noted that lthe output pulse from theoscillator 42is fed to the -grid Aof the gas tube 44, such as a thyratron, V'and in circuit with Athe thyratron tube A44 is Ya gas tube'indicating light 45, such as anordinary neon tube. The function 'ofth'e `t'hyratron 44, rremeriibering that the blocking oscillator npulseoutput has a Vtime duration approximating one microsecond, "is 'to extend `theconduetion 'time of the 'indicating A"light l45 beyond this extremely short time period solthat `the in'- dicating light will appear definitely visible to the human eye. The fsp/ecic vmanner in which the iiniicatirijg'Wl'gl'it is used is best reserved for discussion 'durin'gfatypical operational sequence so, accordingly, arnore detailed "`eX planatio'n of the role 'played by this light willbe :given hereinafter.

As stated above, the blockin'g'oscillatorljiZ'produces `an output pulse 4having a prescribed width, andthe second 'use thereforis now to be mentioned. This pulse is fed, in addition to the 'thyr'atron tube 44,to"a pulsedfosoillator '46, thepurpose of which isto producen forty j mega'cycle pulse having a prescribed pulse widthvvhichstemfs fromV the triggering pulse received from the blocking'oscillator. This'pulse from theoscillator 46 is fedto`a balanced Amodulator 48` which Eis coupled to the local oscillator:cir',h cuitryfalready mentioned but yet to be described;` /T'ne balanced modulator, iikethe mixer L26, any ib'e or the 'hybrid teeor 4short-slot variety.,

power transmitted barok to the Arad r ser 1 6 I ii'oh is undergoing test and to thisiendther'efls employedia cr tal l balanced modulator 48. Also coupled to the balanced modulator 48 is a calibrated variable attenuator 52 which passes the microwave energy to a ferrite isolator 54, the isolator 54 in turn being connected to a transmitting antenna 56. The function of the isolator 54 is to make certain that the energy to be transmitted to the radar set 16 passes to the antennaSli -bu't 'at the same time that the antenna 56 is utilizedonlyfor transmission purposes and does not receive any "signal transmitted by the radar set *16. course"energy'transmitted by the set 16 is to be received by the antenna 22 and notthe transmitting antenna 56 for ifienergywere received byV the antenna 56, then the system would :be disturbed to such an extent that it would not-functipn properly. Inother words the isolator is unidirectional in character.

Having already indicated that it is a desideratum of the invention to simulate diierent ranges, it will now be explained that it is a'lso an aim of the invention to simulate a range rate ofa target. In this connection, as already mentioned quite briey, it is essential to provide a controllable variable time delay to the circuit 40 Aso that transmitted pulses will be relayed back to the radar set 16 after a predetermined lapse of time so as to simulate the range of a target. To simulate the range lrate Aof atarget, however, it is necessary to vary this time delay in a uniform manner. Therefore, returning to the circuit 40 for further description, it is to be pointed lout that the biasing of themultiar circuit constituting the Avariable delay circuit 40 is changed in accordance with a predetermined schedule. To this end a potentiometerj58 Ais utilized, this potentiometer being in the form of a `helipot having its wiper driven in a prescribed mannen lSupplying the potentiometer SS with a reg- 'ulated yD C. voltage'is a source 6). The above-alluded to wiper is driven by a constant speed motor 62 via a speed changer 64. The speed changer 6d may assume avarietyofV different constructions but, suggestively, it may constitute a conventional ball and disk integrator.

.Different range rates are established by setting the lspeed changer to a value corresponding` to any range rate between "200 and 600 knots. To accomplish this, a calibrated dial 66 having a suitable adjustment is connected to the speed changer 64 so that the operator or technician mayl select the particular range rate that is Y c l e sired ft r theradar set 16 undergoing test. Also, as

`when there vis no change in speed by reason of the speed changer 623 being inactivated, the wiper remains stationary and thetarget range is ascertained by virtue of the'particular position of the wiper arm of the potentiometer- 58.' When a range rate is desired, then, of cours e, t hc speec l changer 6 4Ais instrumental in moving 'thepotentiometerswipermarrn 'at a prescribed rate so as to simulate the lactual movement of a target.

If the tester is t o b e capable of performance evalua- 'tion'ofdiffcrent individual radar sets, it is necessary that the tester response Vbelin ear within practical limits over frequency range likely to be encountered by the various radars to be tested. VIt is 'further necessary thatthe tester klystron operate in the dominant mode and at n denite `frequencyfwith respect to that of the Vradarwundery test. `This isjachieved by means of the tuningwandlautomatic frequency control circuitry now to bedesaibed.

It willbe remembered that the balanced mixer 26, through the medium ofLthe gain-stable LF. amplifier 28, `produces pulses thattollow two courses, these pulses in rlsi'ng .the seemed Path homered to 4another LF,- `dz 'rniplifidr 7 '0 thento the limiter and automaticg'lll "coltrl detto'r 'it 72; From the limiter and A.G.C.

ventional Foster-Seeley discriminator type and which:

slaves the local oscillator vfrequency tothe radar frequency during the test, providing, of course, that radarV signals are received in a continuous manner. In order to vascertain when the A.F.C. circuit 74 is tuned, there is provided atuning meter 7 6. lIf the A.F.C. circuit 74:

is at the prescribed forty me'gacycle A frequency, then,

Y of course, there is no output from this circuit'. However,

ifi the frequency is either above or below, then a D.C. voltage of appropriate polarity is sent out depending as far as its-magnitude is concerned upon the deviation in frequency from the desired forty 4megacycles. At

point it'might vbe explained that we have somewhat arbitrarily selected a localoscillator frequencybof forty megacycles above whatever radar frequency is then being-Y encountered, this frequency lending itself readily to practicalutilization. l Y n -The fundamental or primary purpose of the klystron automatic tuning circuitry is to vcause a 2K25 reflex rklystron, or similar klystron, to operate at thepeak of its A power mode and at a frequency which vis in the center of the electronic linear frequency control characteri'stic for this tube where the zero or reference control frequency is prescribed by the operating conditions for the circuit. The klystron tube in this situation has lbeen given the reference numeral 7S appearing in block form i'nFig. 2. purpose for the tuning circuitryv is to enable the opertor totune the klystron 'i8 to a center frequency whichis displaced by the amount Aof the intermediate frequency n from the frequency of the radar to. be tested.

The rellex klystron tuning circuitry presently to be described was devisedy in order to provide ar reflector voltage to the klystron 78 that would produce a maximum power output for a given mechanical tuning, as

already prefaced above when reference was. made to dominant mode operation.

ment soon to be described, the circuitry makes it possible to tune theklystron 78 over its entire vmechanical It might be mentioned that the second By reason of the arrangefy tuning'range without the usual necessity ofy manually Y adjusting the reflector voltage for each mechanical setting .tron tuning takes place are Figs. 3, 4, and 5. Referring new to Fig. 3, there is shown'a portion ofthe output v power plotted aga-inst reector voltage characteristicsfor a reflex klystron. .The several curves representing this relationship bear. the numerals 80a, Stb, and 80e. yMore particularly, it is to be'observed from an inspection of Fig.V 3 that a reflector Voltage yof the value V1 yields a maximum power output whenthe klystron is mechanically tuned to a setting designated by the letter a whereas a. reflector voltage V2 is required to obtain amaximum power jat a setting 15.v It is to be noted that the klystron will not oscillate at the setting b with a reflector voltage having a value V1. Therefore as the klystron is mechanically tuned over its entire frequency range, it is necessary that the reliector voltage be varied in a particular manner in order to make sure that a maximum power output is always obtained.

Directing attention now to Fig. 4, the klystron power output -is plotted as a function of reflector voltage for a' particular mechanical tuning, this curve being represented Of helpin understanding the Ymanner in whichl the klys-V y Whennthe ayeragegreetor voltage is ofthe value V0, the power modulation component will repeat itself each half cycle of the reflector modulating voltage. This signal will not cause output from thephase comparison circuittto be mentioned later) Vand will be referred to as being zero. The power modulation component may thus be utilized as an error signal in4 a servo loop which isto be `constructed in such -a manner so as Ito vary the reilector voltage as the value V0 yields maximum power. f

In describingwhat happens, vattention is directed Vto both Figs. 2 andS. We-'will assume in this'respect that an initial mechanical klystron tuning a and reflector. v voltage 'VR has beenobtained, as indicated by the curve 9l). Under these assumed conditions, power ouput isv a maximum and the frequencyfcomponent'of the output power is zero. Now ifthe klystron 78 istuned by its Y mechanical adjustment 4schematically indicated at 92 to the mechanicalxsetting b,y its power output with respect to its reflector voltage is etectivelygshifted horizontally as indicated/in Fig. 5 by the dotted line curve90a. It is now that an error signal is generated by themodulation in Y the power output which is in phase opposition to the re- Y flector modulation voltage.

Here the superimposed A.C. voltagesignal is indicated by the numerals 94 and 96,

the out-ofphase relationship'being readily discerned byv comparing the resulting curves 94a and 96a.

Inasmuch a's the balanced modulator 48 receives power from the klystron 78, a sample of this power may be taken from the balanced modulator and is first fed to an amplier 98. This signal is graphically depicted in Fig. 5, having been given :the reference numeral 96a.

error signal coming from the phase comparison circuit` 100, rotationV in one direction occurring when thesignalsl are out of phase in one sense Iand an opposite rotation Ytaking place when the signals are out of phase in an opposite sense.` The servo motor 102is'mechanicallyjgeared to a potentiometer 104 across which a D.C. voltage is impressed from a source 106 and whose output is passed through an isolation stage 108 tof the reflector of the :klystron 73, together wtih the reflector modulating signal which is at this time still present, not yet having been removed. For the particular change in the klystron mechanical tuning from a to b illustrated in Fig. 5, the servo motor rotation is such that the rellector voltage lis increased so that it' approaches a value VR for which value the power will be a maximum and the error signal will then return tovzero. Y .Since the reflector voltage is derived from a servo motor driven potentiometer, it will be apparent that the desired klystron output frequency, once attained by means of mechanical tuning, permits the reiector voltage modulating signal to be removed. The optimum reflector voltage, however, will be retained. This is accomplished by opening the previously closed switch 110.

It is the klystron 78 that produces the local oscillator frequency which is lfed to both the balanced mixer 26 and the balanced modulator 48. The tuning circuit just described makes it possible to tune the klystron 78 over a' with the A.F.C. circuit 74 disconnected by having opened a switch 112. The procedure lfollowed in checking when this frequency condition has been reached involves re-v ducing the klystron frequency, the LP. difference from the radar frequency being shown by the tuning meter I rst `indicating a vdeflection ,of one'l polarity followed byai.

is approached. This4 IFrom the amplifier 98 the signal proceeds'to a phase comparison` Vcircuit 100, andif there -is no difference in phase, no outflectionsoppdsite' in Vpolarity to the tiret-tw ol'itrainedV This rneans'that the klyston frequency is now'40 mega;

cycles below-radar` frequency. TheAFC. cannot main# ofV lthe;*radar-ffrequency,` the automatic-tuning circuits? maintainingmaxirnum output at the dominant `model dur-` igr'fthi's mechanical tuning.

-`(v6) "nie-Arcgcirit 74h15 "thnswitehdinby des@V t ing switch`i11f2 A-andfthe400 c`.p.s. modlatinrgvltage froiri-l` thewsouree 895which operates 'the automaticituning servoA loop'iis @switched outl'bysimply*wopeningftheswitch 1105 Bybeatin'g the local' oscillator frequency `irl dwell-known' superbeterodyne-V fashion, Ythe hereinbefore *forty megatain the klystron at this frequency.becausefthcontrol '1"cyclfLFFpulse--isdbtained curve is yoppositeto'thatreduireth and A*afcrmditionof frei l quency instability'resultsi Thus "byobserving aY-proper-f be "assuredf ItY isthen.` ,thatxthe modulating signal fis rel' opening the switch-110 andthe" intermediate frequency is rrieasurementisachievedbefunderstood.` Accordingly, it moved from yth'e klystroni'ellector yoltage by freason of -l mayb'e'eiiplained `at this-time that the amplitude-comi` y parison circuit 36'-plays`an important role`4 this achieve#y rnentjfor nthe detectori 34 puts out lapulse "having fa ma'gthereafter maintained "electronically throughthe agency* Employment of a re'se'trcircuit 114 providesthefklysl 1 tron73 with a rellector voltage prior to the previously alluded to mechanicaltuning'that resultsin the vobtainingy of the' proper mode of klystron oscillation. With theV klystron 78"mechanic'al1`y tuned for its highest frequency oscillation, the reflector voltage is automatically driven by the potentiometer 1434 toa value slightly greater thanh that necessary for oscillation. The reflector voltage is then made tofdecrease until oscillation is obtained. `While power is delivered to a Vdirectional coupler 118. It is` the!l directional coupler thatis employed for forwardingthe power to both the balanced mixer 26 and the balanced modulator 4g. ln this respect it will be recalled that in the early part of the description `it was mentioned thatf an automatic frequency controlled local oscillator was" involved in the circnitryfor supplying the `balancedmixer 26 and later on in the course of the description it wasV mentioned that this'local oscillator also furnished power to the balanced modulator'll. At thisstage ofthe de-l sc'ription',` it is believed evident that the klystron 78, to-

gether .with its associated circuitry, comprises the `local oscillator means. -V i In describing a typical" operationalf sequence of the radar tester' 10, itwill be assumed that'rthe klystron 'n/ has been placed in operation inV accordance with the* above outlined procedure'A and that this local oscillator is delivering a continuous wave output involvingthe radar frequency plus forty megacycles. However, a Vbrief review of the steps taken in tuning the local osciliator will undoubtedly be helpful: v

(l) Mechanical tuning is set forthe highest available frequency with the A;F.C. circuit 74 'disconnected by means of'opening theswitch 112 and'a 400 c.p.s. modu-V lating voltage is applied'to the klystron 'reflector from the source 89;

(2) The reset circuit 114 drives lthe potentiometer so that a maximum negative voltage is applied to the klys` tron reflector.

(3) 'A-limit switch reverses the; potentiometer drive such that'therelector voltagewill approach froma'high' (negativeyside down to the value required to "operate the klystron initsdominant mode. Y' Y (4) Whenthis value is reached and the klystronbegins oscillation, the reset circuit 114 is deactivated, andV the' automatic tuningV circuit drives the potentiometer suchV inasmuch" asV weare 'interested in determining l the oftans'mitt'd radar-"power received? fromA the rlafrfsetv `-16fit is rnpo'rtant'thatthemeans bywhich this ntd Proportional to? the received radar pulse 'p intro-'' d duced into the tester 10 by way of the VantennaVr and the fiitedL attenuatorV 245 lf .this pulse' coming` from' the detectrj34 is of suilicientfmagitude"itfwillbe larger" thanthereference voltage introducedfby V`thesource 33i4 andv willlcau'se the amplit'de"comparison'circuit 36 `to emita large; output pulse'by regenerative` action" when and only when the amplitude of the pulsefrom the detec tor 34fisi`ineitces's of the voltagesupplied by the-DC."

4While the'output'pulse'fromthe amplitude comparison* circuitmightbe feddirectlyto the thyratron'li, none# thel'ess since Vother aims are to be accomplished, -ashere-' f in mentioned, the pulse action is kby way of the delay cir` cnittliandthe blockingoscillatoritZ.v At anyrate itis byvvirtue of alpulsecomingfrom the` circuit 36 that the thyratron tube-44 is fired and such an occurrence is instrmental infindicating that a pulse'has been `emitted i owing to the energizati'on ofthe light 45.

voltage. only variable but calibrated, the LF. gain is Vconstant and known,` and with all other attenuations being lfixed and knowriQthecalibrated reading of this attenuator Si) ypro# vides'anfaccurate measure of thereceived radar transmit*- ted` power. While the provision of Athis measurement will depend, of course, on the stability of the LF. amplifiers 2S and 32, the attenuators 24 and 30,*the reference volti age Supplied at38; as well as the mixer 26, nonetheless,

these Vfactors are well within the control of good design practice andl no particular problem Vis therefore' introthat the klystron 78 isoperatedin'thecenter of its power (5) The operator then'shifts thefrequency of-therdo'm'- inant mode downward byV manual mechanicalA tuning, using theadjustrnent 92,'Iuntil thetuningmeter 76, which indicates-g the ARC. discriminatorV voltage, nshowsf'that a duced.

As for checking' the'target range of the radar set 16 being tested, this is accomplished very readily byf adjusting the speed changer 64 -so that the potentiometer 58 introduces aparticular bias to the electronic delay circuit 4t). The range indicator 68, of course, will signify the particular range corresponding to the delay action of the circuit 40, but of course converted into distance. To set the range indicator, the potentiometer 581 isV driven (through the speed changer 64) by the constantl speed motor" GZ'untilthe desired range is registered on the indicator 68, and the speed changer is disengaged or unclutched from the potentiometer 58. Inasmuch as a delay of one microsecond is equivalent to a distance of approximately 164 yardsV it will be recognized that the delay interval is readily convertible into distance, as is done in calibrating thera'ng'e indicator 68;

Due to' the' fact that the tester 10 is also intended for use in .cliecltingwhether'the computing mechanism associated"` with f the radar".se't -161` isf functioning' Properly;

@there is a need for varying the range in order "t obtain' an indication sto whether the-radar computerlmechaj-v vnisrn is functioning properly. This feature is realized by adjusting the range rate setting at 66 so that a partlcular rate of movement of the wiper arm of the potentiometer 58 is obtained. By controlling in a uniform manner the change inbiasproduced by the potentiometer 58, it will, of course, be understood that the delayv incorporated into the circuit 40 is modified. Such a change in delay rate simulates the change in target range, thereby representing actual movement of the target at a controlled or uniform speed.

Further, the radar vtesterll'i is capable of measuring the .receiverf sensitivity of the Vradar set 16 undergoing test and this particular feature, as far as its operationrsv 'to tire the thyratron tube'44.) This same output pulse which is of-a predetermined width, namely a width cor-` responding-to one microsecond,is fed to the pulsed oscillator46-which produces a forty megacycle pulse to be delivered to -the balanced modulator 48.V ,The modulator 48 seryes'the purpose of combining the forty megacycle pulse1fromthe pulsed oscillator 46 with the local oscillator'signal produced byv-thegreex klystron 78. y It is this modulated wave coming from the balanced modulator that, is 'delivered to the transmitting antenna 56 to provide aradar signal to be sent to the radar set 16.

Howeyen it is important to understand that the tester Y 'is' capable. ofm'easuring the magnitude of power so transmitted, 'To this end, itis to be appreciated that the output of vthebalanced modulatorconsists primarily of the two sidebands of thelocal oscillator produced by the forty megacycle modulation. The lower sideband is at the radar frequencyand is the Only signal-actually rel ceived by the radar set 16. However, the output from thejbalanced modulator 48 .also includes power at a frequency forty megacycles above the radar frequency-as well power at 80 megacycles abovefthe radar frequency. l Y

AWhen measuring receiver sensitivity as we are now concerned with, the variable attenuator 116 is adjusted until the. crystal monitor microammeter 50 provides a current reading of a specified value. This, in effect, sets testlrnade possible with the instant apparatus, then,j ,of

course, the craft on which it is mounted can be immadiately released vand its turn around time is to a very marked extent. v

-As many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended'that all matterl contained in the above description or shown in the accompanying drawings shall be interpreted as illustrativej `and not in a limiting sense.

iIt is also tobe understood that the language used in vthe following claims is intended to cover all of the generic 1 and specific features of the invention herein described and all statements of the scope of the invention which, as amatter. of'language, might be said to fall therebetween.

What is claimed:

1; A radar tester comprising means for producing'a pulse proportional in amplitude to the peak of a'received radar signal, means for providing a reference voltage offpredetermined valuegmeans for comparing said pulse with said reference voltage of predetermined value, means for generating a second pulse only when said first pulse f the-modulator sideband output power to a known specific After adjusting. the attenuator'll, as above outlined, the variableattenuator 52 is then slowly inereaseduntil the attenuation introduced into the circuit is sufficient t0,

cause the radar set 16 to fail to discern the transmitted signal. yStated somewhat differently, the variable attendiffers in -value from said reference voltage, and indip eating means responsive to the second' pulse for indicating whether sa-id second pulse is above or below said reference voltage. f Y

2. A rad/ar rester in accordancewith. claim iin which.

said indicating meansfincludes an indicating light, the

tester further includinga gas tube in circuit with -said v light for supplying electrical energy to said light when said gas tube is tired, said second mentioned pulsebeing a responsible for firing said gas tube. Y 3. A radar tester comprising an antenna Ifor receiving a vtransmitted radar signal pulse, means including a detector for producing a pulse proportional in magnitude to said signal pulse, a calibrated attenuator located in?` termediateI said `antenna Iand `detector for attenuatingfa signal derived from saidantenna and passing therethrough-. l

to saiddetector, means for comparing said proportional kpulse with a reference signal Aof predetermined value toproducelan' output pulse indicative of whether said prog Y portional pulse is greater in amplitude than 'said referkence signal, `and indicating means responsivetofsaid out; put -pulse for signifying the presence of an output pulse,

whereby-adjustment ofv said attenuator will provide an indication of the power received via said antenna when. pulsefr'om said comparison circuit. f

a thyratrontnbein circuit with said indicating means,r g said indicatingl means comprising a-gas tube indicating light, whereby an` output pulse Vfrom said comparison -r'neanswill cause'iiring of said thyratron tube andconi comitant energi'zation of said gas tube indicating light.

5. Aradar tester comprising an antenna for receiving a transmitted radar signal pulse, a iixed radar frequency possible with the radar tester forming Vthe subject matter of the present invention can readily be appreciated and understood. vShould the radar receiving 'set 16 fail in any of the above-enumerated steps, then, of course, the particular trouble is narrowed down and further testing of the components ofV the radar set can be undertaken. The point and advantage to be emphasized atthis time,

however, -is that the aircraft or .other vehicle on--Whichi th'eI-ra'dar set is mounted is not delayed or kept from active "duty lthrough a detailed investigationvof the opera- 1 tional tne'ssof the various components making up ythe raidarlset. the radar receiving set passes the'general attenuator, an automatic frequency controlled local oscil--vl \1ator`, a miirer coupled to said attenuator and local oscillator for producing an intermediate lfrequency pulse, f means `forl amplifying said intermediate frequency pulse, a calibrated attenuator for attenuating said intermediate' frequencj pulse, a detector for producing an output pulse Y for saidintermediate frequency pulse having an amplitude proportional tothe peak of said radar signal pulse, a comparison 'circuit for comparingthe amplitudev of said detector output pulse with a pre-set reference voltage to produce an output pulse from. said comparison circuit when said detector output pulse is of greater amplitudesA thanV said reference voltage, a delay circuit for producing an output pulse at a predetermined "time internal;V afterreceipt of a pulsefrom said comparison ,circuits and a blocking oscillator'l for producing a pulse of predei said'in'dicating means has just begun to'receive an output:

4. A radar'tester in accordance with claim 3 Iincluding 1 l termined-widtli upon receipt of a pulsel from `said delay cil-cuu v 'A 461A rada'r tester in accordancelwith' claim15"including a thyratron tube coupled to said-- blocking'oscillator and'a gas-tubeindicating light in circuit with said thyrato'nlttubel Y i Y 7.`A radar' tester'- in accordancewith claim" 5 including la pulsed oscillator' coupled to said clocking/oscillator, a-nrodulator connected to said pulsed oscillator and said localoscillator, a-'caliora'ted variable attenuatorcoupled to said modulator, a crystal current monitor for measuring powerilowing from said modulator to said variable attenuator, 1 and a transmitting antenna for transmitting` radar"signals Vfromsaid last mentioned attenuatorto aradar set undergolngt'est.V

Y8.A radartester in accordance with claim7 includingl a ferrite isolator disposed between said last mentioned attenuator and said transmitting antenna.

9.' Afradar* tester in accordancewithgclaim 5 including indicating means and transmission means, saidtransmissionmeans' comprising a pulsed oscillator andi amodulater coupledtto said pulsed oscillator and said local oscillator; afp'ulse fromsaid blocking oscillator triggering both'said indicating'means and said pulsed oscillator.

10. A 'radar tester comprising means Afor receiving a rad'r signal pulse, `aidelay circuit'for producing a pulse atati-predetermined interval of time after receipt of aradarV pulse, a'pnlsed oscillator operableV to produce "a plselliavin'gf a selected frequency, a local oscillator for poviding a' carrier wave frequency, a modulator forV combining said oscillator pulse and saidcarrier'wave; a

calibrated .variable attenuator, a crystal current monitoriV fr measuring power owing from said modulator to said attenuator,Y and means coupled to said attenuator for transmitting radar signals toa radar set undergoing test.

1`1. A radar tester comprising `an antenna'for receiving a`--transmitted radar pulse, a local oscillator, means for mixing the radar pulse with theoutput from said local oscillator Vto produce an intermediate frequency signal,y

m'eans` for amplifying the intermediate frequencyV signal,

a calibrated variable attenuator for adjustingV the attenua` tionofsaid'intermediate frequency signal, a detector for dem'odulat'lngV said intermediate frequency signal to' pro'- deeapulse propoitionalin amplitude to .the radarV pulse, means Afor'comparing the outputpulse from said detector with a:known reference voltage, an electronic delay cir- -cuit for producing a pulse at a. predetermined time Vinterval afterreceipt of a Vpulse from said comparison circuit, a

blocking oscillatorconnected to said delay circuit topro- V signal,H and means for transmitting said radar signal.

l2.- A radar tester in accordance with' claim ll in which:

said local oscillator includes a variableattenuator, and thereA is Vfurther included in the tester a'crystal current monitor ammeter for measuring the current output of said modulator, and a calibrated variable attenuator intermediate said modulator and said transmitting means.

13. A-radar tester in accordance withclaim l2.includ ing a ferrite isolator in advance of said transmitting means.

:14. A radar tester comprising a cavity resonator, aV

variable' attenuator coupled to said cavity resonator, a directional coupler connected to said attenuator, a modulat'or' forA receiving power from said coupler, a pulsed oscillator for feeding a pulse having a predetermined freqncyformodulatingthe output from said coupler,V

monitoring means for providing ansindication of the magnitude of power from said'modulator, acaIibrated' attenu ator coupled torsaidmodulator and an.antenna.for-:trans-`- mitting the signalreceived via saidA calibrated attenuator. 15. A-radarteste`r in accordance with claim 14 includi the frequency of said cavity resonator. v

16.'l Alradar tester in accordance with claim 14 in which said cavity resonator is a reflex klystron, the tester; including circuit means for providing'a reflector voltage to l said klystron such thatfor a given mechanical tuningth'e intermediate frequency signal,

control means for said LF. signal, a source of DLC. reflector voltage for said klystron, a source of relatively" power output is a maximum, switch means for cuttingl out saidcircuit means after the maximum power output has been realized, automatic frequency control meansifor controlling the frequency of said klystron,` and switch means for cutting in said automatic frequency'control'- means. l

v 17. A radartester comprising a rellex klystron, a variable attenuator coupled to said klystron, a directional` coupler connected to said attenuator, a balanced modulator for receiving a signal from said coupler and combin ing said signal with a pulsed signal to be transmitted to the radar set undergoing test, a balanced mixer also connected to said coupler forI mixing the signal fromsaid coupler with an incoming radar signal to produce an smallAC. voltage for superirnposition upon said D`.C;

voltage, means for comparing the phase relationship `lie-- tween said A.C." voltage and the A.C. component thereof' appearing in the output of the klystron, servo motor-` means responsive to the phase comparing means, said phase comparing means determining the direction of" rotation of said servo motor means, and means driven by said servo motor means for controlling said reflector voltage in accordance with the degree of phase unbalance' between said A.C. voltage and said A.C. component appearing in the klystron output.

18. A radar tester comprising means for producing a` ing `automatic frequency controlmeans -for controllingh automatic frequency-y signal derived from a radar signal received from a lradar` setl undergoing test, means fortransmitting a radar signal" back to the redar set after a delayed intervalV of time,

means for indicating `when said derived signal is abovea` predetermined Apower level, means responsive to` said derived signal for triggering said transmitting means into operation only if said received radar signal is above said predetermined power level, means foradjusting saidinterval of time to a desired value, and means for measuring the power of the signal transmitted to said radar'v set.

19. A radar set in accordance with claim 18 including means associated with the adjusting means for causing said adjusting means to adjust said interval of time at al uniform rate.

20. A radar tester comprising an atenna, local oscillator means including a klystron, means for'mixing a radar f signal received from a radar set via said antenna with the outputV4 from said local oscillator means to produceL an intermediate frequency signal, means for first establishing a dominant mode operation of the klystron, meansA for automatically controlling the frequency of said klystronfafter the establishment of the dominant mode, means for amplifying said intermediate frequency signal, acallbrated variable attenuator coupled to said amplifying means, a detector for producing an output proportional: to the received radar signal, and amplitude comparison means for comparing the detector output with a fixed reference voltage.

21. A radar tester in accordance with claim 20 in which said comparison means produces an output pulsev when` the detector output is greater than said reference voltage,,

the tester also including means indicating the presence of an output pulsefrom said comparison means.'

22. A radar tester in accordance with claim20`in which f said comparison means produces an output' pulse' when the detector output is greater than said reference voltage, the tester also including a blocking oscillator triggered by the delay means to generate a pulse having a predetermined width, a pulsed oscillator coupled to said blocking oscillator -for producing a pulse having a predetermined frequency, a modulator coupled to said local oscillator 4means and said pulsed oscillator for producing a radar signal, and means for transmitting the last mentioned radar signal back to said radar set.

23. A radar tester in accordance with claim 22 including means for providing an indication of the strength of said last mentioned signal.

24. A radar tester comprising an antenna for receiving a transmitted radar signal, means for producing a signal representative in magnitude to said received radar signal, means providing a reference signal of predetermined value, means for comparing said representative signal with said reference signal of predetermined value to produce an output signal indicative of whether said representative signal is greater than said reference signal, and indicating means responsive to said output signal for signifying the presence of an output signal.

25. A radar tester comprising an antenna for receiving a transmitted radar signal pulse, means for producing a pulse proportional in magnitude to said signal pulse, means providing a reference signal of predetermined valve, means for comparing said proportional pulse with said reference signal of predetermined value to produce an output pulse indicative of whether said proportional pulse is greater in amplitude than said reference signal, and indicating means responsive to said output pulse for signifying the presence of an output pulse.

26. A radar tester comprising means for producing a derived signal from a radar signal received from a radar set undergoing test, means for indicating when said derived signal is above a predetermined power level, means for transmitting a radar signal back to the radar set after a delayed interval of time, means responsive to said derived signal for triggering said last-mentioned means into operation only if said received Vradar signal is above said predetermined power level, and means for adjusting said interval of time to a desired value.

27. A radar tester in accordance with claim 26 including means associated with the adjusting means for causing said adjusting means to adjust said interval of timeat a uniform rate.

28. A radar tester comprising an antenna for receiving a transmitted radar signal, means in circuit with said antenna for introducing a known amount of attenuation, means connected to said attenuating means for producing a derived signal, means for providing a reference signal of predetermined value, means for comparing said Aderived signal with` said reference signal to produce an output signal indicative of Whether said derived signal Y is greater in magnitude than said reference signal, and indicating means responsive to said output signal for signifying the presence of an output from said comparison means.

29. A radar tester in accordance with claim 28 in which said attenuating means includes a calibrated variable attenuator, whereby adjustment of said attenuator will provide an indication of the power received via said antenna when said indicating means has just begun to receive an output signal from said comparison circuit.

30. A radar tester comprising means for receiving a signal transmitted from a radar set undergoing test including means -for attenuating the received signal a predetermined amount, means for producing a derived signal from the attenuated signal, means for indicating when said derived signal is above a predetermined power level, means for transmitting a radar signal back to the radar set after a delayed interval of time, means responsive to said derived signal for triggering said last-mentioned means into operation only if said received radar signal is above said predetermined power level, and means for adjusting said interval of time to a desired value.

References Cited in the le of this patent UNITED STATES PATENTS Deloraine et a1. May 27, 1947 

